Since its discovery in the 1950s, olefin metathesis has emerged as a valuable synthetic method for the formation of carbon-carbon double bonds. In particular, its recent advances in applications to organic syntheses and polymer syntheses mostly rely on developments of well-defined catalysts. Among attempts to improve catalyst efficiency over the past decade, one of the most attractive frontiers has been selective synthesis of stereo-controlled olefin product. Derived from generally accepted their equilibrium reaction mechanisms, most of catalysts give higher proportion of thermodynamically favored E isomer of olefin in products. This fundamental nature of olefin metathesis limits its applications to some reactions including natural product synthesis. Thus, a catalyst which selectively gives Z isomer of olefin product is expected to open a new convenient route to a value-added product. Especially, use of Z selective catalysts in olefin cross metathesis (CM) is promising for outstanding methodology in organic chemistry. In the simplest case of such CM, two different terminal olefin molecules selectively generate one new internal cis-olefin molecule and one ethylene molecule (Scheme 1).

One of the most important classes of olefin metathesis catalysts is ruthenium-based alkylidene complex represented by the ruthenium catalyst (1-4) (FIG. 1). Because of their high efficiency in catalysis and high tolerance towards various functional groups, they are most widely used in both academic and industrial fields. Typical ruthenium catalysts are known to give more E isomer than Z isomer in CM and other olefin metathesis reactions (see Chatterjee, A. K.; Choi, T.-L.; Sanders, D. P.; Grubbs, R. H. J. Am. Chem. Soc. 2003, 125, 11360).
Bielawski et al. reported that a ruthenium catalyst having acyclic diaminocarbene ligand (5) afforded the cross coupled product in a nearly 1:1 ratio of its E and Z isomers at high conversion (˜75%) in CM of allylbenzene and cis-1,4-diacetoxy-2-butene (see Rosen, E. L.; Sung, D. H.; Chen, Z.; Lynch, V. M.; Bielawski, C. W. Organometallics 2010, 29, 250). Grubbs et al. also demonstrated that a bulky sulfonate ligand substituted 2nd generation catalyst (6), which was readily prepared from commercially available reagents, gave the product with E isomer/Z isomer=2.9 at very high conversion (˜90%) in the same CM reaction (see Teo, P.; Grubbs, R. H. Organometallics 2010, 29, 6045). Compared to the original ruthenium catalysts, these catalysts gave much more Z isomer of the product; however, their Z selectivity were still not satisfactory for precisely stereo-controlled reactions. On the other hand, some of the molybdenum- or tungsten-based catalysts recently developed by Hoveyda and Schrock are outstanding for their Z selectivity in metathesis homocoupling of terminal olefins (see Jiang, A. J.; Zhao, Y.; Schrock, R. R.; Hoveyda, A. H. J. Am. Chem. Soc. 2009, 131, 7962). In a particular case, bulky aryloxide substituted tungsten catalyst (7) afforded homocoupled product of 1-hexene with 95% Z isomer. Despite the excellent Z selectivity, their relatively many synthetic steps and generally required strict reaction conditions for molybdenum and tungsten alkylidene catalysts somewhat restrict their use in common organic syntheses.
In order to overcome the above mentioned disadvantages of the current catalysts, new highly Z selective ruthenium based catalysts are needed. For general use, especially in industry, they should be not only tolerant towards various functional groups and impurities in reaction media but also readily synthesized from common reagents in simple reaction steps. Despite the advances achieved in preparing olefin metathesis catalysts, a continuing need in the art exists for improved catalysts, including catalysts that provide improved Z selectivity.